Lithium-ion secondary battery is a high-performance secondary battery and has been used in various applications such as cell-phone, notebook computer and camcorder because of its high energy density, and is expanding its market significantly. Lithium cobaltate or lithium manganate is generally used as the cathodic electroactive material and graphite as the anode in smaller lithium ion batteries among them. Such a lithium-ion battery also contains a separator of a porous sheet such as of polypropylene, polyethylene, or the like, and an electrolyte solution, for example, an organic solution containing a lithium salt such as an ethylene carbonate-based solution of lithium hexafluorophosphate (LiPF6).
More specifically, the cathode of common lithium-ion secondary batteries is prepared by immobilizing a cathodic electroactive material such as lithium cobaltate or lithium manganate and electron-conductive-carbon fine particles having a current-collecting effect for conveying electrons therefrom (thereto) on a metal foil. The metal foil used then is generally an aluminum foil, and, for example, polyvinylidene fluoride (PVDF) or polytetrafluoroethylene (PTFE) is used as the binder for immobilizing the cathodic electroactive material and carbon fine particles thereon.
Recently, there is an increasing trend toward application of such a high-performance secondary battery in the fields demanding power such as automobiles, which gave rise to problems not foreseen from conventional smaller batteries.
One of them is quick discharge/recharge characteristics. A greater amount of current is needed for generation of greater power. For that reason, a battery, which loses its capacity rapidly, should be recharged once again. Recharge of the battery should be shortened by using a greater current, because a longer recharge period results in elongation of the period when the battery cannot be used. The discharge and recharge characteristics at a greater current, jointly referred to as quick discharge/recharge characteristics, are an important indicator of secondary battery usefulness.
As described above, a greater current is indispensable for quick discharge and recharge. However, discharge and recharge of conventional lithium-ion secondary batteries at a greater current cause an inconvenience of drastic deterioration in capacity (retention rate of its initial battery capacity) when the discharge and recharge are repeated, i.e., deterioration in output power during repeated discharge and recharge at a greater current. More specifically, it is currently almost impossible to discharge and recharge a battery at 20 C (i.e., at a current 20 times greater than that needed for discharging and recharging a battery in one hour) even if it is possible to discharge and recharge battery at 1 C (a current allowing discharge and recharge of a battery in one hour); and thus, various attempts, for example those described in the following literatures, were made to improve such a problem.    Japanese Unexamined Patent Publication No. 2001-266850    Japanese Examined Patent Publication No. 7-123053    Japanese Patent No. 1989293    45th Battery Symposium (2004) 3C18
However, the methods described in respective literatures were not sufficiently effective in repair the nonconformity.
The description herein of advantages and disadvantages of various features, embodiments, methods, and apparatus disclosed in other publications is in no way intended to limit the present invention. Indeed, certain features of the invention may be capable of overcoming certain disadvantages, while still retaining some or all of the features, embodiments, methods, and apparatus disclosed therein.
Other objects and advantages of the present invention will be apparent from the following preferred embodiments.